Many advancements have contributed to a recent surge in phase change memory development. With reference to FIG. 1, one recent improvement that has resulted in a simplified and lower cost method of manufacturing phase change memory cells is the inclusion of a switch 124, such as a selectable diode or an ovonic threshold switch, together with a phase change memory storage element 122 in a stacked memory cell 120 of an apparatus 100. Adding the switch 124 within each stacked memory cell 120 eliminates the need to form a transistor switch in the semiconductor substrate below or above each respective memory cell 120.
In order to prevent heat transfer between the phase change memory storage element 122 and the switch 124, however, a thermally insulative electrode 130 such as carbon is typically positioned between the phase change memory storage element 122 and the switch 124. The carbon electrode 130 provides good electrical conductivity (for voltages and currents to pass through), but inhibits the transfer of thermal energy between the phase change memory storage element 122 and the switch 124. The carbon electrode 130 may also serve as a diffusion barrier to prevent diffusion of materials between the phase change memory storage element 122 and the switch 124 during manufacture and operation of the memory cell 120.
Some manufacturing processes of depositing the carbon 130 (or other material), such as physical vapor deposition (PVD), may cause the carbon 130 to form in a columnar manner. Such a columnar carbon electrode 130, however, provides a poor diffusion barrier because the columnarity allows various materials to diffuse across the electrode 130. For example, oxygen, indium, selenium, and so forth may diffuse from the phase change memory storage element 122 to the switch 124, or vice versa, during manufacturing—for example during deposition, etching, thermal cycling and annealing, and/or electrical cycling—or during operation of a finished and packaged memory device. This diffusion of various materials across the electrode 130 may lead to degradation and, eventually, to premature failure of a memory device.